1. Field of the Invention
The present invention relates to a power conversion apparatus used in an inverter and a power conversion system having the power conversion apparatus.
2. Description of the Related Art
An inverter applied to general industrial machines or automobiles generally has a power conversion apparatus which drives a motor (load). When the inverter is, for example, a three-phase inverter, the power conversion apparatus includes insulated gate bipolar transistors (referred to as an “IGBT” hereinafter) in the phases U, V, and W, free wheel diodes (referred to as an “FWD” hereinafter) each of which is connected to each IGBT by back-to-back, and a device control circuit which controls these power devices (One power device consists of one IGBT and one FWD.). The device control circuit generally includes an interface circuit which exchanges signals with a host control circuit, a drive circuit which drives power devices, a protection circuit which detects an abnormality such as overvoltages or overcurrents of the power devices to protect the power devices from breakdown. The protection circuit has resistors to divide a voltage (referred to as “voltage-dividing resistors”, hereinafter) and a comparator. The comparator compares the value of a signal output from each of various sensors with a predetermined reference value (a value of a voltage generated by a reference voltage and the voltage-dividing resistors). The protection circuit outputs a signal designating to cut-off the gate of the IGBT or the like when the value of the signal output from each of various sensors exceeds (or is lower than) the reference value. By this protection circuit, the IGBT and the motor can be prevented from being broken by overheat, overcurrent (short-circuit or overload), and overvoltage of the IGBT, a decrease in the voltage of a power supply of the device control circuit, and the like.
Important characteristics of the above-mentioned power conversion apparatus are a switching loss at turning-on and turning-off of the IGBT and a surge voltage at turning-off. The switching loss (ESW (ON)/ESW (OFF)) is expressed by a product between an output voltage (collector-emitter voltage VCE) and an output current (IC) of the IGBT. The switching loss means an energy loss in a switching operation of the power conversion apparatus, therefore, it is desirable to minimize the switching loss.
The surge voltage (Vsurge) is defined by an equation (1) by using a parasitic inductance L generated by the internal and external wires of the power conversion apparatus, and a rate of a change of the output current IC of the IGBT (OFF) at switching off it.Vsurge=dIc(OFF)/dt×L  (1)The surge voltage affects the withstand voltages of the IGBT and the FWD. Under conditions that value of the output current IC is equal, areas occupied by the IGBT and the FWD on a chip increase as the withstand voltages thereof decrease. For this reason, from the viewpoint of cost, it is desirable to minimize the surge voltage. The trade-off is established between the surge voltage and the switching loss.
For example, some conventional inverter includes two inverter circuits connected in parallel to each other, and a control circuit for controlling output voltages of these inverter circuits by a phase control signal. The inverter detects the frequency of a surge voltage at an output voltage obtained by synthesizing the two output voltages, and correct the phase control signal by using the detected frequency to suppress the surge voltage (refer to, for example, Laid-Open Japanese Patent Publication No. 10-225130).
Generally, the output voltage (collector-emitter voltage) VCE of the IGBT and the output current IC thereof are determined by characteristics of the IGBT and those of each component of a drive circuit to drive it. And, those characteristics are independent with each other. Thus, the output voltage and the output current vary depending on a variation in each component, a variation of characteristics of each component by temperature change, aging, or the like. The variation of the output voltage and that of the output current causes the variation of the switching loss and that of the surge voltage. When the variations of the switching loss and the surge voltage are excessively large, the characteristics of the power conversion apparatus are deteriorated, and the power conversion apparatus and the motor load erroneously operate in some cases.
The Laid-Open Japanese Patent Publication No. 10-225130 or the like discloses an inverter to suppress a surge voltage. However, it does not mention the deterioration of characteristics caused by an increase in switching loss.
Also, the variation of a predetermined protection level in the protection circuit deteriorates a protection function, and therefore degrades the characteristics of the power conversion apparatus. This is due to that the protection level is predetermined, but actually varies as an output voltage VCE of an IGBT, an output current IC thereof, and the characteristics of detection circuits therefor vary depending on the temperature change, aging, or the like, even though the condition of controlling them is same. Therefore, in some cases, the IGBT is detected to operate abnormally, even though the IGBT does not abnormally operate. Conclusively, the power conversion apparatus disadvantageously performs an erroneous operation.
As a result, the conventional power conversion apparatus cannot avoid deterioration of its characteristics.